Ownership: What Happens When the Diesel Exhaust Fluid Tank Runs Dry? We Put It to the Test

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As if it weren’t tough enough for diesel owners to find a clean place to pump fuel, they face another challenge: keeping their tanks of diesel exhaust fluid (DEF) topped off. What happens if you run the treatment tank dry? We tested to find out, so you won't have to try it yourself.

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Diesel veterans long ago learned how to handle this concern. The following primer is for diesel-engine newbies and gasoline-engine devotees curious about life on the other side. DEF, also known as AdBlue or Blue DEF, is an elixir that's necessary to make sure that what exits a diesel vehicle’s tailpipe is as sweet as an Alpine breeze. Those who turn a DEF ear face harsh penalties.

DEF, available at most service stations for $6 to $10 per gallon, is a fluid consisting of urea and deionized water in a 1:2 solution. It’s contained in a reservoir separate from the fuel tank and is metered into the engine’s exhaust stream to control certain emissions. Inside the exhaust pipe, the DEF vaporizes and decomposes into ammonia and carbon dioxide. Those two compounds then react with oxygen and detrimental nitrogen oxides inside the vehicle’s selective catalytic reduction (SCR) catalyst. This produces three benign tailpipe gases: nitrogen, water vapor, and carbon dioxide.

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Make sure that any DEF you purchase is labeled to comply with ISO (International Organization for Standardization) 22241 specifications. While DEF doesn’t deteriorate over time, it can freeze, so store your supply in a heated location. The ingredients remain in solution, so no shaking or mixing is necessary before use.

What Happens When the Diesel Exhaust Fluid Tank Runs Dry? We Put It to the Test© Don Sherman What Happens When the Diesel Exhaust Fluid Tank Runs Dry? We Put It to the Test

The Environmental Protection Agency requires makers of new light-duty diesels to interrupt the engine’s normal startup routine if the DEF runs out to make sure owners take their clean-air responsibilities seriously. To avoid the problem of owners stranded at the side of the road, every diesel-vehicle maker provides ample warning before it’s time to add DEF.

The two long-term diesel-powered vehicles currently in the Car and Driver test fleet are proving useful for studying DEF consumption scenarios. Here, we’ll focus on the 2016 Land Rover Range Rover HSE Td6 we’ve enjoyed driving more than 30,000 miles since last spring, but we’re also keeping watch over a 2016 Nissan Titan XD powered by a 5.0-liter Cummins V-8.

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Our Range Rover is equipped with a 3.0-liter turbo-diesel V-6 that sips DEF from a 4.8-gallon tank located under the floor beneath the driver’s seat. That reservoir’s fill cap is under the hood atop the left fender. According to the owner’s manual, a full DEF tank should last 6300 miles, but, as everyone knows by now, your mileage will vary.

While there’s no DEF gauge in our Range Rover (some vehicles have those now), the driver can determine how many miles remain before this juice runs out by calling up the service menu in the display located between the tachometer and speedometer. With the engine off, under the Next Oil Change heading, there’s a notation reading “DEF refill XXX miles.”

We soon discovered that the earliest warning is fleeting and easy to miss. According to Land Rover, the first message—“Diesel exhaust fluid level low”—occurs at around 1500 miles before the no-restart doomsday. We apparently missed that message, but at 969 miles before the end, one of our editors on a weekend trip noted: “First DEF warning spotted. There was no orange triangle in the cluster display, and an inattentive driver would have missed it. The warning disappeared before I could photograph it.”

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Running a Diesel out of DEF© Provided by Car and Driver Running a Diesel out of DEF

The next alert appeared 254 miles later. This time, there was a bright orange triangle surrounding an explanation point accompanied by the “Fill diesel exhaust fluid tank” message captured in the photo above.

Only 215 miles later, the urgency intensified. In addition to the warning above, a new message threatened, “No engine restarts possible in 500 miles.” We continued driving and watching without adding one drop of DEF. No additional messages lit up our cluster while we were driving. But during the normal starting routine, the advisory read: “No engine restarts possible soon. Fill diesel exhaust fluid tank. Distance to NO ENGINE RESTART 100 miles.” Again, we added no fluid.  But we did pay heed by switching off the Range Rover’s automatic stop/start function and by mapping a route close to the Car and Driver garage.

After exactly 100 miles, the stern warning “No engine restarts possible” appeared in the driver’s display cluster. We kept the engine running to drive the final eight miles to the comfort of our garage before shutting it down.

To make sure the Range Rover wasn’t kidding, we attempted to fire its engine but found no sign of life under the hood. As we had been warned, without DEF our diesel refused us service, as if we'd walked into a 7-Eleven without shoes or shirt. We promptly began resuscitation procedures. Adding 2.5 gallons of DEF did not—to our surprise—revive the patient. The engine would not start until we tipped in another 2.3 gallons of juice, filling the DEF tank to its brim. Then this diesel started without hesitation, idled nicely, and ran perfectly throughout its next journey—a 250-mile jaunt to Chicago.

While the frequency of warnings does vary, all modern diesels give their owners plenty of notice that their DEF must be replenished well before the dry-tank doomsday occurs. Our Titan, for example, has a gauge that’s handy for checking the DEF level before embarking on a major trip.

This experiment convinced us that keeping track of your DEF is no more difficult than monitoring the engine’s oil level. There are ample warnings, and the Range Rover’s supply tank is large enough to last most drivers several months. Considering how effective this strategy is for cleansing the exhaust, the half-cent per mile we spent in this test for DEF is quite reasonable.

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